Pretty much all ISPs allocate dynamic IPs, which may change when you reboot the modem or may change at intervals, or pretty much whenever they feel like it. Part of that may be to discourage running home servers - most ISPs do also have a section in their terms & conditions prohibiting this.
As an alternative, consider a free dynamic DNS service such as DynDNS or no-ip - they let you have a domain name which "tracks" your IP, so if your IP changes, the domain name will still get you there. That means you could have, say, myteamspeak.dyndns.com and everyone would just point their TS client to that instead of 111.111.111.111. The free services will have restricted features, I think the one I sued back when I ran a home server only gave you the domain for 6 weeks at a time or something, but it's hardly a hassle to update it every few weeks or months.
Hope this helps.