Is this an external serial modem, or an internal Win-modem? External should work fine, internal is "iffy".
You should be able setup connection from the Networking section. Not at Mint box now to give precise directions, sorry. But where it enables you to play with network setting in Control panel (or whatever name), should also see and allow you setup dialup also. Have a look./ Also, let us know what modem type it is. I don't want to go googling for obscure models, etc. Too tired, and nearly off to bed.