Randomly hear signal wile waiting for answer


i use DIal with r option in my dialplan.
funny thing is that using the same hardware phone (phones) i sometimes hear signal
when waiting for answer and sometimes do not head any signal before somebody answers.

Using softphone (idefisk) i always had that signal.

Where might be the problem ? Why using hardware phones (SIP between phone and asterisk)i sometimes hear signal while waiting for answer, sometimes not ?