Asterisk 1.4 with Multiple SIP trunk

Hi all

i am registered to a SIP provider. the problem is, each sip account is only capable of making 1 sip calls. to enable 5 calls, i would need to register 5 accouns.

now my questions is, is there a way to have all 5 SIP accounts to register in asterisk as one trunk ?

more like zap channels, when one zap is not available it will fall to the next available trunk.

you hint and instruction is much appreciated