That was a common feature in the old days where buttons represented physical phone lines. As you know, phone "lines" are now virtual in the SIP world and I do not believe there is a way to assign a specific trunk to a virtual line on your phone.
The most common approach when you want a call to go out a specific trunk is to use prefix digits. What I mean by this is on your outbound rules specify that to match that route the call must begin (as an example) with 7777. Then, when you dial the phone number you "prepend" digits to the number. For example, if I wanted to dial 8016424655, I would dial 77778016424655. The outbound rule should be configured to strip the 7777 from the call before the call is routed to your SIP provider. Naturally, the outbound rule specifies the desired trunk to use, and can also specify the outbound caller ID number to use.
This approach is straightforward when you have only a few different trunks, but could get a little less easy to use if you had to remember a large number of prefixes. In your case it sounds like 7777 for one trunk, 8888 for another, etc.
