Any old DSL is not specific enough since there are so many variants of DSL out there that would not be compatible.
For legacy DSL any ADSL modem should work (but you may want to future proof your purchase with an ADSL2+ capable modem).
For single line Fusion you would need a modem that supports ADSL2+.
For bonded (dual line) Fusion you would need a modem that supports the bonding of two ADLS2+ lines which you are unlikely to find in a retail store.
The advantage of getting Sonic to provide you with the modem is that they will take care of it when it needs to be updated or replaced. It also makes troubleshooting easier for their support staff (which will get you up and running faster). The downside is the monthly modem fee instead of the one-time purchase price of a modem.--
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