Well both actually, depending on what it's doing. On routine or training flights, Portland's SAR helicopter (an AW139 for those in the know) takes the call sign Coastguard 106. (said One-Ohh-Six)
Once it's on a "shout" i.e. a proper rescue mission, it adopts the tactical call sign Rescue 106.
Previously the helicopters were known by the last two letters of their registration marks - WB, or Whiskey Bravo - but nowadays the numeric call sign refers to the operating base.
The photo above is of G-SARD 'Romeo Delta' returning to Portland on Sunday, and as such its call sign was Coastguard 106. Normally, G-CGWB 'Whiskey Bravo' is Portland's aircraft, but currently 'Romeo Delta' is covering.
|G CGWB (Whiskey Bravo)|
You may hear us refer to Coastguard 104 or Rescue 104 - that's when we're working with a helicopter from Lee on Solent, perhaps because 106 is otherwise engaged or 104 is already airborne and can respond quicker.