Telnet offers business grade services. Caribserve does not.
If you do have a phone line, ADSL from Telnet is more reliable and can be a lot faster (depending on the package you choose). Wireless Broadband, either from IDL, Caribserve (I believe now UTS), or Telnet is about the same quality.
Also, Telnet offers the option of a fixed ip address, this is useful if you want to set up a server
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