Two VOIP systems on one Internet connection?

I am having trouble digging up answers to my question - even my ISP tech support couldn’t answer it.
I work from home and have an office extension phone (VOIP) on my home office desk. I want to replace my personal phone with VOIP, which obviously implies two different VOIP systems in my house. The question is, is this possible. My router directs incoming ports 5060 to 5063 to my office extension. So, I think part of the solution, if there is one, is to have signalling to the second VOIP system come in on ports different from 5060 to 5063. Problem is, no one I have approached so far seems to know. Strangely enough, tech support at my ISP (from whom I was going to purchase a VOIP home phone service) could not tell me which ports my router would need to forward to the VOIP adapter that they supply.
My apologies in advance as I am sure I have not framed my problem well. Any assistance or direction would be very much appreciated.
Best regards,

Your ITSP is probably a black box operator that doesn’t really know how their equipment works and can only handle simple questions. Either that, you you failed to get beyond the first line support person, who is generally working from a script.

5063 is not a standard IP phone port number.

This doesn’t seem to involve Asterisk. Although you could interpose Asterisk, this is not the type of case you would want to attack with little knowledge.

Routers can try to be clever with SIP. If you are trying to be clever, you need to disable this and be more clever with the phones.

Assuming that the router is not too clever, you could try defining a non-standard SIP port number on one of them and port forwarding that appropriately. The router needs to route outgoing SIP and RTP based on the destination address.

If you are luck, at least one side will be prepared to look at the actual port number used in your REGISTER request and your outgoing calls, rather than insisting an a fixed port number. Just to be clear, unless the other side imposes policy constraints, you choose the local port numbers and IP addresses when you need to register with a SIP server. Only if you are not using registration does the ITSP need to know or care about this information.

For a home user, your employer’s IT department are likely to be more flexible, and a better source of help, than your ITSP.

if you are looking at having to multiple clients behind a nat connecting to a remote server (from an itsp for instance), it is usually possible as many clients select a random port - bear in mind that if you are contacting your itsp on its ip:5060, there is rarely any issue if your client is initiating the dialog from yourip:anyport

ata like ht503 actually let you choose if you want a random source port or to specify one. As long as your client registers and sends regular packets, your firewall will keep this random port opened and you should be able to receive incoming calls.

Thanks David44 and Cerien.Jean.
My VOIP expertise is a little less than nil. Although, when our office went to VOIP and I was given the option of having an office extension in my home office (150 miles from my official workplace), it was I that figured out how to get it working. Mostly trial and error but based on some advice I found on the Internet that ports 5060-5070 (depending on the number of lines required) needed to be forwarded to the VOIP phone and some high number ports needed to be open.
Learning to manage an asterisk server is not high on the list of things I want to do. I was just seeing it as a possible solution. However, after the bafflegab I got from my ISP tech support, who simply said that it could not be done but could not explain why, and the company that installed our office system, it is apparent that Internet telephony management is not for the faint of heart.
Am I understanding correctly that it is actually the VOIP phone that defines the port that it is to be contacted on? If that is correct, then there should be no real limit to the number of different systems working on one Internet connection. Mine is dynamic DSL, if that makes a difference, and I have a DynDNS with my own host name.
I am thinking that the best route might be to take my ISP up on their trial VOIP offer and see what it takes to make things work.
Best regards,