Obviously a critical part of a GSM relay board is the actual physical relay connected to the output. Advent Controls use both Omron and Songle brand relays depending on availability. We’ve found both to be of high quality and reliability and both units match or exceed the 10A 240AVC/24VDC specification.
What is critical in the design of these products is allowing sufficient creepage distance between the potentially high voltage COM, NO and NC terminals and the DC parts of the circuits. Similarly, although the actual distance is lower, is the distance between conductors – COM and NO or COM and NC. Our design conforms to a minimum 4mm clearance between the ground plane and output terminals and 2mm clearance between conductors allowing our boards to be used for switching mains powered equipment.
There is an excellent article regarding creepage and clearance (where isolation slots are machined into the PCB) on the following website: http://www.ce-mag.com/ce-mag.com/archive/01/03/ProductSafety.html