ISDN or T1 or modem

I have a delimma here.

One of my office is revamping its entire phone system since it sorely out of date and because it has so few lines in comparison to the number of staff there. Since there is no knowing when the revamping will take place we like to have other alternatives for connecting to main office email system.

Shall we proceed with ISDN or T1 or dedicated phone lines?